Warming waters likely to affect jellyfish numbers in Atlantic Canada

Jellyfish congregate off the Halifax waterfront boardwalk. Climate change could cause us to see less of the gelatinous species in the coming years. - Eric Wynne

A day at the beach in the Atlantic region often begins with the question, how many jellyfish are out there?

The answer to that question may change as our ocean waters, particularly in areas closer to land get warmer amid climate change.

“Anecdotally, the last few years (people have) been seeing less jellyfish than they used to,” said Beth Nordstrom, a graduate student in marine biology at Halifax’s Dalhousie University, in a recent interview.

That would make sense because when Maritimers talk about jellyfish, they’re likely picturing the big purple blobs known as the lion’s mane, the most common species in the region. Unlike most jellies, the lion’s mane is partial to colder water.

“So they’re one of the few species that people are actually thinking might not do well with warming ocean temperatures,” said Nordstrom, who is finishing up her master’s thesis on the connection between jellyfish populations and one of their most avid predators, the endangered leatherback turtle.

Besides updating the status of the leatherback population, Nordstrom’s research will fill a big gap in our sparse knowledge of jellyfish distribution and numbers.

“For a long time, people weren’t really interested in monitoring jellyfish populations,” said Nordstrom, who’s collaborating with Mike James of the Department of Fisheries and Oceans on her thesis, which is being supervised by Boris Worm at Dalhousie University.

“Part of it is they don’t really do a lot to benefit us as humans, we tend to monitor fish stocks or charismatic species like whales.”

Since beginning her study two years ago, she has asked citizen scientists to report jellyfish sightings at jellyfish@dal.ca with the location, description and date of the sighting. About 90 per cent of those reports have been on the lion’s mane, she said.

Another common species in our region is the moon jellyfish, a translucent creature between 25 and 40 centimetres wide, which have four horseshoe-shaped rings (which are their sexual glands) at the top of their body.

And then there’s the comb jelly, also known as sea gooseberries. These delicate creatures, about the size of a large grape, are often mistaken for baby jellyfish. They’re actually not true jellyfish: they are classified as ctenophora while the scientific name of jellyfish is medusozoa. But combs are often lumped in with jellyfish because they’re gelatinous and eat plankton, Nordstrom said.

With the exception of cold water species like the lion’s mane, jellyfish populations are expected to increase across the world as our oceans get warmer. Global ocean temperatures already have increased by 1.4 F since 1901, according to the Environmental Protection Agency in the United States. Warmer water holds less oxygen and jellyfish can survive in lower oxygen environments, which gives them a competitive advantage against other species in expanding their range.

What will more jellyfish mean for ocean ecology in general? Obviously, it will benefit jellyfish eaters like the leatherback and may give a much-needed boost to this giant reptile’s numbers. Other jellyfish lovers include the ocean sunfish, which Nordstrom called a “very strange-looking fish,” which can grow upwards of 1.8 metres and weigh more than 2,000 kilograms.

But jellyfish are predators themselves, mainly of plankton, the tiny creatures that are also the staple of whales and many fish species.

“They’ll be eating more zooplankton and fish larva, and that will impact the rest of the food chain,” Nordstrom said.